Coordinated Multi-Point (CoMP) transmission/reception has been proposed as a promising technology to meet the Third Generation Partnership Project (3GPP) Long Term Evolution Advanced (LTE-A) requirements by improving performance of cell-edge UEs in particular. In CoMP operation, multiple transmission/reception points cooperatively transmit to or receive from one or more user equipment (UEs) to improve performance, especially for those UEs that would otherwise, in the case of downlink, see significant interference from some transmission points if they do not cooperate. A transmission point (TP), termed from the downlink perspective, refers to generally a radio unit controlled by the scheduler in a base station (referred to as eNodeB or eNB in LTE). A base station may control a single TP, in which case the TP is the same as a base station or an eNB. In this case, the CoMP operation refers to the case that there is coordination among eNBs. In another network architecture, a base station or eNB may control multiple TPs that are often referred to as radio units or radio heads. In this case, coordination among TPs will happen naturally, and is easier to achieve since they are controlled by a centralized scheduler within the eNB.
In some network deployments, TPs may be co-located, in which case it is feasible to connect them to a single eNB. An example is the well-known three-sector deployment where a single eNB has three service areas referred to as sectors or cells. In some other deployments, TPs may be geographically separated, in which case they can be controlled by either separate eNBs or a single eNB. In the former case, TPs are also typically under the control of separated schedulers that may coordinate in a peer-to-peer fashion. Different types of eNBs with different transmission powers constitutes the so-called heterogeneous network. In the latter case, the TPs, often referred to as remote radio units (RRUs) or remote radio heads (RRHs), connect to a single eNB via optical fiber and a centralized scheduler controls/coordinates all the TPs.
Each TP, co-located or geographically separated, may form its own logical cell or multiple TPs may form a single logical cell. From a user equipment (UE) perspective, a cell is defined as a logical entity that a UE receives data from and transmits data to, in other words, “serves” the UE. The cell that serves a UE is called the “serving cell”. The geographic area covered the logical entity is sometimes also referred to as a cell, such as when a cell-edge UE is mentioned to describe a UE located at the edge of the coverage area. A cell usually has an associated cell identifier (cell-ID). A cell-ID is typically used to specify the pilot signals (also referred to as reference signals) and scramble the data transmitted to the UEs “attached” to (i.e., served by) that cell.
At a high level, two general CoMP categories can be used to describe coordination schemes. First, two or more TPs can jointly transmit to a user in the so-called Joint Transmission (JT) scheme. JT schemes turn otherwise interference into constructive signal for the UE, while also achieve the spatial multiplexing gains due to a larger set of antennas. Second, in the so-called Coordinated Scheduling (CS) approach, two or more cells can coordinate dynamically such that their transmission such that the cross interference is reduced when they serve multiple UEs simultaneously. One simple CS example is called Dynamic Cell Selection (DCS) where the best cell for transmission is selected dynamically on a short-term basis. Further muting of the transmission from some cells to completely avoid interference will further improve SNR at the user. In another example referred to as Coordinated Beamforming (CoBF), the spatial domain of the multi-antenna channels is exploited by eNB to choose spatial precoding or beamforming weights appropriately at both serving and non-serving cells.
There are pros and cons for JT and CS, which take different approaches to interference management. For example, JT requires dynamic exchange of data among all the participating TPs, which may be feasible only to architectures that have a centralized scheduler with fiber-based connection to TPs. For CoBF, one of the drawbacks of relying exclusively on spatial domain interference mitigation is that the performance improvement may be limited by the accuracy of spatial channel feedback. This is often the case where the uplink constraint limits the feedback to be quantized codebook based feedback.
DCS requires dynamic coordination among cells, even though it does not need to exchange a lot of data as in JT. Traditionally, cells are connected through a standardized higher latency interface referred to as X2, which is not designed to enable dynamic coordinate on a short-term basis. The information exchanged over X2 is often coarse and based on long-term channel characteristics and traffic loads etc, hence only a slow coordination among distributed schedulers is possible. As an example of coordination, each cell may adopt an associated subframe muting pattern according to pre-planning or slow coordination.
CoMP operation via fast coordinated scheduling is of particular interest to the network architecture where a centralized scheduler controls multiple TPs via fiber based connection.
In conventional non-CoMP operation, a single TP, which is the serving cell for a number of UEs, adapts the transmission parameters based on the quality of the links to those UEs. To support link adaptation, a technique commonly adopted in modern wireless communications, a UE needs to estimate the channel quality of a hypothetical data transmission which is traditionally from a single cell. Channel quality is often represented as a modulation and coding scheme (MCS). The UE may also feed back some recommendation on the spatial transmission parameters, such as the transmission rank indication, precoding matrix index, and the like.
In CoMP operation, transmission from multiple points also needs to adapt to the link condition as seen by the UE. The key to realize fast scheduling gains is for each individual TP to respond to the traffic loading and user channel and interference condition dynamically with adaptive resource allocation, user selections, muting and power management. CoMP needs to be enabled by appropriate UE feedback which also should reflect the performance improvement resulted from CoMP.
The various aspects, features and advantages of the invention will become more fully apparent to those having ordinary skill in the art upon careful consideration of the following Detailed Description thereof with the accompanying drawings described below. The drawings may have been simplified for clarity and are not necessarily drawn to scale.